Laboratory research on behavior analysis proved to be useful in establishing principles of learning with many relevant applications for people. Early efforts in the applied behavior analysis area proved to be particularly successful with mentally retarded persons. Self-help skills received much of the earliest attention, but another area that became quite fruitful for study was dual diagnosis--mental health problems of mentally retarded individuals. This paper reviews some of the early works of Skinner and his colleagues and the implications of this work on the rapidly developing subdiscipline of dual diagnosis. Current status and future trends are discussed.